When a conventional vane type air motor is brought to a stall, it produces an erratic torque output due to the unpredictable friction effects and random positioning of the blades. A similar effect is experienced with certain types of electric motors. When these motors are utilized as the drive means for rotating and applying a torque to a fastener the resulting torque on the fastener is unpredictable and erratic. A method of overcoming this problem has recently been developed. The method allows the motor housing to rotate slowly backwards at stall, restrained by a smooth running viscous clutch. In this manner the motor repeatedly applies its maximum output thus substantially overcoming the effect of friction and random positioning at stall.
The major objections to allowing the motor to rotate are the added costs of mounting such a motor and making air or power connections to it. A somewhat similar effect can be obtained by installing a slip clutch device between the motor and the output spindle either before or after the gear reduction normally required. If the clutch is placed in line after the gear reducer the clutch must be made large enough to carry the full output torque. In either event, the added rotating mass of the clutch creates an inertial force which must be absorbed upon stall of the fastener. In certain types of hard joints, this results in overtorquing or even failure of the fastener.